This invention relates to flexible polyurethane foams and a method for their preparation. This invention particularly relates to flexible polyurethane foams requiring reduced amounts of chlorofluorocarbon blowing agents.
Flexible polyurethane foams are used in a wide variety of applications. Such applications include bedding, carpet backing, packaging, toys, furniture cushion manufacture, and the like. Use of flexible polyurethane foams in automobiles is one such application and a very important one. In automobile manufacturing, flexible polyurethane foams are used to prepare seat cushions, arm rests, head rests, knee buttresses, and the like.
Flexible polyurethane foams used in automobile manufacturing function in at least two ways. The first is to provide comfort. Flexible polyurethane foams can be prepared to provide variable load bearing strength. Foams with relatively low load bearing strength are perceived as being "soft". Seating is often perceived as being more comfortable when the seating is soft. However, when the foam around that soft section has a higher load bearing strength, one sitting on such a cushion is held in place against sudden lateral motions, the cushion thereby also providing support. By varying the load bearing strength of the foams in automobile seat cushions such that the seats are softer in the middle and firmer at the sides, automobile seats can be prepared which provide both comfort and support. Load bearing strength is varied by altering the ratio of the two primary components of a polyurethane foam, a polyisocyanate component and an active hydrogen containing component. This ratio is also known as the foam's isocyanate index or simply, index.
Second, flexible polyurethane foams function to contribute to automobile safety. Use of foam in, for example, knee buttresses, contributes to automobile safety because the foam absorbs kinetic energy from an object impacting it. In the event of an automobile collision, where a passenger's knees impact a knee buttress, energy is absorbed by the foam as it deforms and lessens the effect of impact on the passenger, thereby reducing or eliminating injury.
Traditionally, flexible polyurethane foams have been prepared from toluene diisocyanate (TDI) formulations. TDI is prepared by phosgenating toluene diamine. The phosgenation reaction product is distilled to produce TDI monomer and a residue which is typically discarded. TDI has a functionality of about 2.0.
A relatively recent trend in the production of flexible polyurethane foams is to prepare them from methylene diphenyldiisocyanate (MDI) formulations rather than TDI formulations where applications require physical properties not attainable with TDI, or where the lower vapor pressure of MDI makes it a desirable alternative to TDI. MDI is prepared by phosgenating polymethylene polydianiline. The phosgenation reaction product is often distilled to produce MDI monomer and a residue containing polymeric MDI known as PMDI. PMDI is sometimes directly produced without a distillation step by phosgenating polymethylene polydianiline to produce a PMDI with desirable viscosity, isomer ratio, or other physical properties. PMDI is often useful in applications where material of higher functionality is required, such as the production of rigid polyisocyanurate foams. The functionality of PMDI can exceed about 3.0.
Rather than using MDI or TDI monomer to produce flexible polyurethane foams, a polyisocyanate, particularly polymethylene polydiphenyldiisocyanate, or a prepolymer can sometimes be used instead. A prepolymer is the reaction product of a di- or poly-isocyanate and an active hydrogen containing compound, such as a polyol or polyamine. Prepolymers can be prepared from formulations having an excess, that is, more than a 1:1 equivalent ratio, of either the active hydrogen containing compounds or the di- or poly-isocyanate. Prepolymers prepared from formulations having an excess of active hydrogen containing compounds are described as OH- or NH- terminated and are further reacted with additional di- or poly-isocyanate to prepare a polyurethane. Prepolymers prepared from formulations having an excess of di- or poly-isocyanate are described as ISO- or NCO- terminated and are further reacted with active hydrogen containing compounds to prepare a polyurethane.
U.S. Pat. No. 4,261,852 to Carroll, et al. discloses preparing a liquid polyisocyanate composition by blending PMDI and an MDI prepolymer. This patent describes prepolymers prepared from formulations having polyether diols or triols, or blends of polyether diols and triol, of 700 equivalent weight or greater.
Foams are prepared by frothing a liquid matrix which afterwards solidifies into a solid having many voids. In preparing a foam, a source of vapor or gas must be provided in order to froth the liquid matrix. Materials which produce or act as these sources of gas or vapor are called blowing agents. A blowing agent is a material which vaporizes or otherwise produces gas during the forming of a foam and thereby serves to reduce the density of the foam. Chlorofluorocarbons have long been used as blowing agents for flexible polyurethane foams. However, due to environmental considerations, the use of chlorofluorocarbons has been increasingly discouraged and use of alternative blowing agents has been found to be desirable.
An example of an alternative type of blowing agent is carbon dioxide. Foams produced from formulations having blowing agents which generate carbon dioxide are considered environmentally superior to foams prepared utilizing only chlorofluorocarbon blowing agents. Water reacts with isocyanates to produce carbon dioxide and is an example of a carbon dioxide producing blowing agent.
While it has been found desirable to reduce the use of chlorofluorocarbon blowing agents, it has not always been trouble-free to do so. Chlorofluorocarbons often contribute to the physical properties of the foams produced therewith. For example, foams produced from formulations having carbon dioxide generating blowing agents rather than chlorofluorocarbon blowing agents often have poorer insulative and flammability properties. Generally, reducing chlorofluorocarbon blowing agents in favor of carbon dioxide generating blowing agents results in a flexible polyurethane foam with poorer physical properties such as tear strength, tensile strength, and elongation.